THE FINAL ADJUSTMENTS IO5 



waste products. What they actually do is to convert 

 the nitrogen into fairly insoluble substances, such 

 as urea, or, in some highly evolved animals, aric 

 acid, which is kept in a special organ until the 

 embryo escapes from the shell and can excrete it. 

 In the development of birds, for instance, there is a 

 very striking example of chemical recapitulation in 

 this respect; in the early stages the nitrogen is 

 excreted as ammonia, as it is in fish, then it is con- 

 verted into urea and finally, at the end of develop- 

 ment, most of the waste nitrogen is being formed into 

 uric acid. Birds and reptiles have also become 

 modified to produce less nitrogenous waste ; they do 

 this by using more fat and carbohydrate and less 

 protein, as fuel, and they get the added advantage 

 from this that the fat and carbohydrate give rise to 

 some water when burned and thus help with the 

 provision of the necessary moisture. 



So far nothing has been said about the feeding of 

 the mammalian embryo. This is because of the quite 

 special and peculiar conditions which are involved 

 in uterine life. In a sense, the mammals have found 

 a way of returning their embryos to the sea, the 

 "sea" in this case being the maternal blood, which 

 can supply oxygen, water, and salts, and can remove 

 waste products, and which has the additional 

 advantage of supplying carbohydrates, fats, and 

 proteins as well. 



The essential feature of the evolutionary step taken 

 by the mammals is to place the embryo inside the 

 body of the mother and to bring the embryonic and 



